* Video Player now acting as a parent that always knowing the state of
  their children.
* Events on the sub-controls are now triggered on itself, and binded by
  the Video Player instead of triggering everything on the Video Player.
* A new helper class, SubView, has been introduced to cleanup repeat
  logic on scoped jQuery selector, render() and bind()

Begin using a Keystore for XML parsing. Still broken: sequence icons, custom tags, problems, video js

From https://www.pivotaltracker.com/story/show/31700967
Fixes #137

* use clearer names for the two status strings passed to js
* add functions to do canonical conversion of progress to js string
* fix updating bug in sequence.coffee
* add some todo comments and other docs to make future expansion easier

* change bottom border of links:
  red for not started
  yellow for in_progress
  green for done

* This should probably be designed at some point.
* Obvious problems: the yellow is not very visible, and lots of people are
  red/green color-blind.

* add module_from_xml param to I4xSystem
* use it to implement xmodule.get_children()
* fix a few comments here and there
* Render-time progress display for seq and vertical modules.
  - Computes fraction of subproblems done.
* Pass problem state back to js during ajax calls.
* general cleanup in capa_module.py
* add progress_changed and progress fields to json returned from each ajax handler

* Coffeescript changes to hook up sequence tracking of problem progress

* net result: sequence 'a' tags now have a progress class
* properly set css class on initial load
* fire event when progress changes after ajax calls
* also save state in 'progress' property of problems-wrapper tag
* event handler finds those tags, computes updated progress

Using `.ui-slider-handle` was too broad.

This will leave out the student that never attempt the question and
fixes the error with histogram rendering.

There might be a chance that an error got thrown after 'contentChanged'
event was fired and stop the `toggleArrow()` method to be called. This
will make sure that the navigation will still be working even some
element in the content is failed.

   - responsetype used to be instantiated multiple times(!) in capa_problem
     now it is instantiated once, and stored in self.responders
   - responsetypes.GenericResponse restructured; each superclass
     show now provide setup_response (and not __init__), and may
     provide get_max_score(); general __init__ provided to
     clean up superclasses.